The polymerization of THF by oxonium ion catalysis became known as the result of the basic work by H. Meerwein et al (Angew, Chemie 72, (1960), 972) and is described comprehensively in the monograph "Polytetrahydrofuran" by P. Dreyfuss, Gordon and Breach Sc. Publishers, New York, London, Paris 1982.
U.S. Pat. Nos. 3,433,829, 4,189,566 and 4,243,799 describe the polymerization of specially purified THF in the presence of a carboxylic anhydride and bleaching earth, which is a naturally occurring aluminum silicates having a cryptocrystalline three layer structure of the montmorillonite mineral. The mineral obtained from deposits exhibits physical and chemical properties which vary depending on the origin. In the international PCT-Application WO 94/05719 an improved process is described by catalysts selected from synthetic amorphous aluminum silicate, acid-activated and calcinated kaolin or zeolite in the presence of carboxylic anhydride. Similar procedures are described in the JP-Applications 4-306228, 4-277522 and the U.S. Pat. Nos. 5,208,385 and 5,210,283 with synthetic mainly amorphous silica-alumina. In particular, the activity of the catalyst is not constant, but instead varies from batch to batch. This is a serious disadvantage for commercial utilization of these catalysts. Besides, only very pure THF can be polymerized.